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Automation of subsea connections for clusters of wave energy converters
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Wave power)
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2015 (English)In: The Proceedings of the Twenty-fifth (2015) International Ocean and Polar Engineering Conference, 2015Conference paper (Refereed)
Abstract [en]

To make wave power a viable energy source, large clusters of wave energy converters should be deployed. For most of the farms the output power of the WECs should be aggregated in a marine substation and then transmitted to the grid. The need for cost effective underwater cable connection operations is one of the main issues in offshore operations. Underwater connections can be conducted with wet- or dry-mateable connectors, performed by divers or ROVs. Although there are existing solutions used by the oil and gas industry that could be employed, the capital expenditure needed is not compatible with the offshore renewable energy industry.


The objective of this research is to decrease costs and minimize working hazards associated with sub-sea work when performing these underwater electrical connections. This article presents a solution using small ROV’s instead of divers to execute the task. The main idea is to perform the connection underwater, but using dry-mateable connectors. A solution to make this possible is to install air pockets at the substation enclosing the connectors. These boxes are meant to be filled with air and hence create a dry environment in which to perform the connections. This is achieved with help of two tools. First a docking system allows the operator to fix the ROV at the substation before doing the connection. Then a gripper tool added to the ROV grasps the cable and connects it to the substation in the air pocket. The procedure and design of this low-cost solution are described, and the different prototypes that have been tested for offshore operation are also shown.

Place, publisher, year, edition, pages
Keyword [en]
Wave energy, ROV, subsea connection, gripper, substation
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
URN: urn:nbn:se:uu:diva-259835ISBN: 978-1-880653-89-0OAI: oai:DiVA.org:uu-259835DiVA: diva2:845679
The Twenty-fifth International Ocean and Polar Engineering Conference, June 21-26, Kona, Hawaii, USA
Lysekil project
Available from: 2015-08-12 Created: 2015-08-12 Last updated: 2016-09-07Bibliographically approved
In thesis
1. Underwater Electrical Connections and Remotely Operated Vehicles
Open this publication in new window or tab >>Underwater Electrical Connections and Remotely Operated Vehicles
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Remotely Operated Vehicles (ROVs) are underwater robots that perform different kind of operations, from observation to heavier tasks like drilling, carrying and pulling cables, etc. Those ROVs are costly and require skilled personal to operate it as well as equipment for transportation and deployment (boats, cranes, etc.).

The division for electricity at Uppsala University, is developing a wave energy converter (WEC) concept. The concept is based on a point-absorbing buoy with a directly driven linear generator placed on the seabed. Several units are connected to a marine substation that is located on the seabed, whose role is to collect and smooth the power absorbed from the waves and then bring it to the shore through one single cable.

Cable connection is a big challenge in the project because the WEC concept is small and many units are necessary to create a rentable farm. Nowadays this operation is performed by divers but using Observation Class ROV (OCROV) could be an interesting alternative since they are affordable at lower costs and easier to operate. Cable connection is however a heavy task and requires force that an OCROV does not have. It will need a docking system from which the vehicle will take its force. It would then go to the station, dock itself to this support plate, grab the cables and connect them together. This procedure cannot be done by the ROV operator because it requires accurate displacement and quick adjustment of the robot’s behavior.

An autopilot was created in Matlab Simulink that consists of three units: the path following, the ROV, and the positioning unit. The first one uses the vehicle’s position and computes the speed and heading to be applied on the ROV in order to guide it on the desired path. The second one contains a controller that will adapt the thrust of each propeller to the force needed to reach the desired heading and speed from the path following unit. It also contains the model of the ROV that computes its position and speed. The last unit consists of a Kalman filter that estimates the ROV position and will be used in case of delay or failure in the communication with the positioning sensors.

The autopilot model is used with a positioning system that utilizes green lasers and image processing. Two green lasers are used as fixed points in each camera picture and from their distance on the image, the actual distance between the ROV and the docking platform can be computed. In addition, optical odometry is used. The idea behind is to estimate how the ROV is behaving by evaluating the changes between two pictures of the camera. Those two systems, laser and odometry, work together in order to get more accurate results.

The laser system has so far been tested in air. The distance measurements gave interesting results with an error inferior to 3%, and angle measurements gave less than 10% error for a distance of one meter. One advantage with the system is that it gets more accurate as the vehicle gets closer to the docking point.

In addition to the ROV project, a review study was conducted on the variability of wave energy compared with other resources such as tidal, solar, and wind power. An analysis of the different tools and models that are used to forecast the power generation of those sources was done. There is a need for collaboration between the different areas because the future will aggregate those different sources to the grid and requires a unification of the models and methods.

Place, publisher, year, edition, pages
Uppsala: Department of Engineering Sciences, 2016. 66 p.
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352 ; 349-16L
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
urn:nbn:se:uu:diva-302644 (URN)
2016-09-30, 16:08 (English)
Lysekil project
Available from: 2016-10-18 Created: 2016-09-07 Last updated: 2016-10-18Bibliographically approved

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